Everyone has a bad first day, and there are three separate reasons stacked on top of each other.
The pack is at its heaviest on day one, all the food and usually the most water. By day three it's two kilos lighter and everything is easier.
You go out too fast. Nearly everyone does, because you're fresh and excited and you've been sitting in a car. Deliberately walking the first two hours slower than feels right makes a dramatic difference.
And you haven't switched over yet. It takes most people about a day to stop thinking in emails and start thinking in weather and water sources. That transition feels like misery and it's just adjustment.
What actually reduces it: a shorter first day. Six or seven kilometres to the first camp rather than eighteen. Costs you almost nothing across four days and changes the tone of the whole trip.